This invention relates to an apparatus and a method for controlling the level of ink in a fountain for offset printing presses.
In offset printing presses, ink is used to transfer images from the printing plate to the impression blanket roller and then to the paper. The ink is stored in a fountain or reservoir and is transferred to a fountain roller which is in intimate contact with the ink. From the fountain, the ink flows to the printing plate via a series of ink train rollers. The consistency of the printed image is significantly affected by the ink film thickness on the ink train rollers. The ink film thickness is controlled, in part, by the amount of ink surface exposure to the fountain. Therefore, constant monitoring of the fountain ink level and make-up of the ink supply is necessary to maintain quality output of the printed product from the offset printing press.
Several techniques have been employed to sense the level of ink in offset printing press fountains, including floats (see U.S. Pat. No. 3,025,798), tactile or mechanical sensors (see U.S. Pat. Nos. 3,373,052 and 3,848,529), pneumatic sensors, capacitive sensors (see U.S. Pat. No. 4,010,683) and ultrasonic sensors. The techniques which require intimate contact with the ink in the fountain (floats, tactile and pneumatic sensors) have proven to be unreliable.
Both capacitive and ultrasonic sensors have been used in a variety of apparatus to measure the level of the ink supply. The capacitive apparatus measures the change in ink level by monitoring the change in the dielectric constant of a capacitor formed using an electrode as one plate and the fountain metal as the second plate. The air and ink separating the plates act as the dielectric.
Apparatus using ultrasonic sensing detects the change in ink level by measuring the time between transmitting a burst of sonic energy from a transducer and receiving an echo of that emitted energy. The echo is the reflection from the surface of the ink. Typically, the transducer consists of a piezo-ceramic element which is used to convert electrical energy to mechanical energy when transmitting and mechanical energy to electrical energy when receiving. Knowledge of the speed of sound in the air at ambient temperature permits calculation of the distance separating the transducer and the ink surface, thus making changes in the height of the ink surface detectable. When the level of the ink surface falls below the desired level or set-point, an electrical signal is generated which causes an electrically actuated valve to open. When the valve is opened, a flow of ink is established from a storage tank to the ink fountain. The level of the ink is measured using the ultrasonic sensor until it rises above the desired level or set-point, then the electrical signal to the electrically actuated valve is turned off, and the flow of ink from the storage tank to the ink fountain is interrupted. The monitoring of the ink supply using an ultrasonic transducer is relatively complex. Factors such as ink surface air bubbles, hills and valleys (disturbances to the surface of highly viscous, unmixed inks will last for long periods of time), large disturbances caused by mechanical ink mixers employed in the fountain and movement of the air separating the transducer and the ink surface can cause the echo reflected from the ink surface to disappear intermittently or to vary considerably from reading to reading. This measurement instability increases the complexity of the control circuitry and software programs (if used) significantly.